syms.texi revision 1.1.1.1
1@section Symbols 2BFD tries to maintain as much symbol information as it can when 3it moves information from file to file. BFD passes information 4to applications though the @code{asymbol} structure. When the 5application requests the symbol table, BFD reads the table in 6the native form and translates parts of it into the internal 7format. To maintain more than the information passed to 8applications, some targets keep some information ``behind the 9scenes'' in a structure only the particular back end knows 10about. For example, the coff back end keeps the original 11symbol table structure as well as the canonical structure when 12a BFD is read in. On output, the coff back end can reconstruct 13the output symbol table so that no information is lost, even 14information unique to coff which BFD doesn't know or 15understand. If a coff symbol table were read, but were written 16through an a.out back end, all the coff specific information 17would be lost. The symbol table of a BFD 18is not necessarily read in until a canonicalize request is 19made. Then the BFD back end fills in a table provided by the 20application with pointers to the canonical information. To 21output symbols, the application provides BFD with a table of 22pointers to pointers to @code{asymbol}s. This allows applications 23like the linker to output a symbol as it was read, since the ``behind 24the scenes'' information will be still available. 25@menu 26* Reading Symbols:: 27* Writing Symbols:: 28* Mini Symbols:: 29* typedef asymbol:: 30* symbol handling functions:: 31@end menu 32 33@node Reading Symbols, Writing Symbols, Symbols, Symbols 34@subsection Reading symbols 35There are two stages to reading a symbol table from a BFD: 36allocating storage, and the actual reading process. This is an 37excerpt from an application which reads the symbol table: 38 39@example 40 long storage_needed; 41 asymbol **symbol_table; 42 long number_of_symbols; 43 long i; 44 45 storage_needed = bfd_get_symtab_upper_bound (abfd); 46 47 if (storage_needed < 0) 48 FAIL 49 50 if (storage_needed == 0) 51 return; 52 53 symbol_table = xmalloc (storage_needed); 54 ... 55 number_of_symbols = 56 bfd_canonicalize_symtab (abfd, symbol_table); 57 58 if (number_of_symbols < 0) 59 FAIL 60 61 for (i = 0; i < number_of_symbols; i++) 62 process_symbol (symbol_table[i]); 63@end example 64 65All storage for the symbols themselves is in an objalloc 66connected to the BFD; it is freed when the BFD is closed. 67 68@node Writing Symbols, Mini Symbols, Reading Symbols, Symbols 69@subsection Writing symbols 70Writing of a symbol table is automatic when a BFD open for 71writing is closed. The application attaches a vector of 72pointers to pointers to symbols to the BFD being written, and 73fills in the symbol count. The close and cleanup code reads 74through the table provided and performs all the necessary 75operations. The BFD output code must always be provided with an 76``owned'' symbol: one which has come from another BFD, or one 77which has been created using @code{bfd_make_empty_symbol}. Here is an 78example showing the creation of a symbol table with only one element: 79 80@example 81 #include "bfd.h" 82 int main (void) 83 @{ 84 bfd *abfd; 85 asymbol *ptrs[2]; 86 asymbol *new; 87 88 abfd = bfd_openw ("foo","a.out-sunos-big"); 89 bfd_set_format (abfd, bfd_object); 90 new = bfd_make_empty_symbol (abfd); 91 new->name = "dummy_symbol"; 92 new->section = bfd_make_section_old_way (abfd, ".text"); 93 new->flags = BSF_GLOBAL; 94 new->value = 0x12345; 95 96 ptrs[0] = new; 97 ptrs[1] = 0; 98 99 bfd_set_symtab (abfd, ptrs, 1); 100 bfd_close (abfd); 101 return 0; 102 @} 103 104 ./makesym 105 nm foo 106 00012345 A dummy_symbol 107@end example 108 109Many formats cannot represent arbitrary symbol information; for 110instance, the @code{a.out} object format does not allow an 111arbitrary number of sections. A symbol pointing to a section 112which is not one of @code{.text}, @code{.data} or @code{.bss} cannot 113be described. 114 115@node Mini Symbols, typedef asymbol, Writing Symbols, Symbols 116@subsection Mini Symbols 117Mini symbols provide read-only access to the symbol table. 118They use less memory space, but require more time to access. 119They can be useful for tools like nm or objdump, which may 120have to handle symbol tables of extremely large executables. 121 122The @code{bfd_read_minisymbols} function will read the symbols 123into memory in an internal form. It will return a @code{void *} 124pointer to a block of memory, a symbol count, and the size of 125each symbol. The pointer is allocated using @code{malloc}, and 126should be freed by the caller when it is no longer needed. 127 128The function @code{bfd_minisymbol_to_symbol} will take a pointer 129to a minisymbol, and a pointer to a structure returned by 130@code{bfd_make_empty_symbol}, and return a @code{asymbol} structure. 131The return value may or may not be the same as the value from 132@code{bfd_make_empty_symbol} which was passed in. 133 134 135@node typedef asymbol, symbol handling functions, Mini Symbols, Symbols 136@subsection typedef asymbol 137An @code{asymbol} has the form: 138 139 140@example 141 142typedef struct bfd_symbol 143@{ 144 /* A pointer to the BFD which owns the symbol. This information 145 is necessary so that a back end can work out what additional 146 information (invisible to the application writer) is carried 147 with the symbol. 148 149 This field is *almost* redundant, since you can use section->owner 150 instead, except that some symbols point to the global sections 151 bfd_@{abs,com,und@}_section. This could be fixed by making 152 these globals be per-bfd (or per-target-flavor). FIXME. */ 153 struct bfd *the_bfd; /* Use bfd_asymbol_bfd(sym) to access this field. */ 154 155 /* The text of the symbol. The name is left alone, and not copied; the 156 application may not alter it. */ 157 const char *name; 158 159 /* The value of the symbol. This really should be a union of a 160 numeric value with a pointer, since some flags indicate that 161 a pointer to another symbol is stored here. */ 162 symvalue value; 163 164 /* Attributes of a symbol. */ 165#define BSF_NO_FLAGS 0x00 166 167 /* The symbol has local scope; @code{static} in @code{C}. The value 168 is the offset into the section of the data. */ 169#define BSF_LOCAL 0x01 170 171 /* The symbol has global scope; initialized data in @code{C}. The 172 value is the offset into the section of the data. */ 173#define BSF_GLOBAL 0x02 174 175 /* The symbol has global scope and is exported. The value is 176 the offset into the section of the data. */ 177#define BSF_EXPORT BSF_GLOBAL /* No real difference. */ 178 179 /* A normal C symbol would be one of: 180 @code{BSF_LOCAL}, @code{BSF_FORT_COMM}, @code{BSF_UNDEFINED} or 181 @code{BSF_GLOBAL}. */ 182 183 /* The symbol is a debugging record. The value has an arbitrary 184 meaning, unless BSF_DEBUGGING_RELOC is also set. */ 185#define BSF_DEBUGGING 0x08 186 187 /* The symbol denotes a function entry point. Used in ELF, 188 perhaps others someday. */ 189#define BSF_FUNCTION 0x10 190 191 /* Used by the linker. */ 192#define BSF_KEEP 0x20 193#define BSF_KEEP_G 0x40 194 195 /* A weak global symbol, overridable without warnings by 196 a regular global symbol of the same name. */ 197#define BSF_WEAK 0x80 198 199 /* This symbol was created to point to a section, e.g. ELF's 200 STT_SECTION symbols. */ 201#define BSF_SECTION_SYM 0x100 202 203 /* The symbol used to be a common symbol, but now it is 204 allocated. */ 205#define BSF_OLD_COMMON 0x200 206 207 /* The default value for common data. */ 208#define BFD_FORT_COMM_DEFAULT_VALUE 0 209 210 /* In some files the type of a symbol sometimes alters its 211 location in an output file - ie in coff a @code{ISFCN} symbol 212 which is also @code{C_EXT} symbol appears where it was 213 declared and not at the end of a section. This bit is set 214 by the target BFD part to convey this information. */ 215#define BSF_NOT_AT_END 0x400 216 217 /* Signal that the symbol is the label of constructor section. */ 218#define BSF_CONSTRUCTOR 0x800 219 220 /* Signal that the symbol is a warning symbol. The name is a 221 warning. The name of the next symbol is the one to warn about; 222 if a reference is made to a symbol with the same name as the next 223 symbol, a warning is issued by the linker. */ 224#define BSF_WARNING 0x1000 225 226 /* Signal that the symbol is indirect. This symbol is an indirect 227 pointer to the symbol with the same name as the next symbol. */ 228#define BSF_INDIRECT 0x2000 229 230 /* BSF_FILE marks symbols that contain a file name. This is used 231 for ELF STT_FILE symbols. */ 232#define BSF_FILE 0x4000 233 234 /* Symbol is from dynamic linking information. */ 235#define BSF_DYNAMIC 0x8000 236 237 /* The symbol denotes a data object. Used in ELF, and perhaps 238 others someday. */ 239#define BSF_OBJECT 0x10000 240 241 /* This symbol is a debugging symbol. The value is the offset 242 into the section of the data. BSF_DEBUGGING should be set 243 as well. */ 244#define BSF_DEBUGGING_RELOC 0x20000 245 246 /* This symbol is thread local. Used in ELF. */ 247#define BSF_THREAD_LOCAL 0x40000 248 249 /* This symbol represents a complex relocation expression, 250 with the expression tree serialized in the symbol name. */ 251#define BSF_RELC 0x80000 252 253 /* This symbol represents a signed complex relocation expression, 254 with the expression tree serialized in the symbol name. */ 255#define BSF_SRELC 0x100000 256 257 /* This symbol was created by bfd_get_synthetic_symtab. */ 258#define BSF_SYNTHETIC 0x200000 259 260 flagword flags; 261 262 /* A pointer to the section to which this symbol is 263 relative. This will always be non NULL, there are special 264 sections for undefined and absolute symbols. */ 265 struct bfd_section *section; 266 267 /* Back end special data. */ 268 union 269 @{ 270 void *p; 271 bfd_vma i; 272 @} 273 udata; 274@} 275asymbol; 276 277@end example 278 279@node symbol handling functions, , typedef asymbol, Symbols 280@subsection Symbol handling functions 281 282 283@findex bfd_get_symtab_upper_bound 284@subsubsection @code{bfd_get_symtab_upper_bound} 285@strong{Description}@* 286Return the number of bytes required to store a vector of pointers 287to @code{asymbols} for all the symbols in the BFD @var{abfd}, 288including a terminal NULL pointer. If there are no symbols in 289the BFD, then return 0. If an error occurs, return -1. 290@example 291#define bfd_get_symtab_upper_bound(abfd) \ 292 BFD_SEND (abfd, _bfd_get_symtab_upper_bound, (abfd)) 293 294@end example 295 296@findex bfd_is_local_label 297@subsubsection @code{bfd_is_local_label} 298@strong{Synopsis} 299@example 300bfd_boolean bfd_is_local_label (bfd *abfd, asymbol *sym); 301@end example 302@strong{Description}@* 303Return TRUE if the given symbol @var{sym} in the BFD @var{abfd} is 304a compiler generated local label, else return FALSE. 305 306@findex bfd_is_local_label_name 307@subsubsection @code{bfd_is_local_label_name} 308@strong{Synopsis} 309@example 310bfd_boolean bfd_is_local_label_name (bfd *abfd, const char *name); 311@end example 312@strong{Description}@* 313Return TRUE if a symbol with the name @var{name} in the BFD 314@var{abfd} is a compiler generated local label, else return 315FALSE. This just checks whether the name has the form of a 316local label. 317@example 318#define bfd_is_local_label_name(abfd, name) \ 319 BFD_SEND (abfd, _bfd_is_local_label_name, (abfd, name)) 320 321@end example 322 323@findex bfd_is_target_special_symbol 324@subsubsection @code{bfd_is_target_special_symbol} 325@strong{Synopsis} 326@example 327bfd_boolean bfd_is_target_special_symbol (bfd *abfd, asymbol *sym); 328@end example 329@strong{Description}@* 330Return TRUE iff a symbol @var{sym} in the BFD @var{abfd} is something 331special to the particular target represented by the BFD. Such symbols 332should normally not be mentioned to the user. 333@example 334#define bfd_is_target_special_symbol(abfd, sym) \ 335 BFD_SEND (abfd, _bfd_is_target_special_symbol, (abfd, sym)) 336 337@end example 338 339@findex bfd_canonicalize_symtab 340@subsubsection @code{bfd_canonicalize_symtab} 341@strong{Description}@* 342Read the symbols from the BFD @var{abfd}, and fills in 343the vector @var{location} with pointers to the symbols and 344a trailing NULL. 345Return the actual number of symbol pointers, not 346including the NULL. 347@example 348#define bfd_canonicalize_symtab(abfd, location) \ 349 BFD_SEND (abfd, _bfd_canonicalize_symtab, (abfd, location)) 350 351@end example 352 353@findex bfd_set_symtab 354@subsubsection @code{bfd_set_symtab} 355@strong{Synopsis} 356@example 357bfd_boolean bfd_set_symtab 358 (bfd *abfd, asymbol **location, unsigned int count); 359@end example 360@strong{Description}@* 361Arrange that when the output BFD @var{abfd} is closed, 362the table @var{location} of @var{count} pointers to symbols 363will be written. 364 365@findex bfd_print_symbol_vandf 366@subsubsection @code{bfd_print_symbol_vandf} 367@strong{Synopsis} 368@example 369void bfd_print_symbol_vandf (bfd *abfd, void *file, asymbol *symbol); 370@end example 371@strong{Description}@* 372Print the value and flags of the @var{symbol} supplied to the 373stream @var{file}. 374 375@findex bfd_make_empty_symbol 376@subsubsection @code{bfd_make_empty_symbol} 377@strong{Description}@* 378Create a new @code{asymbol} structure for the BFD @var{abfd} 379and return a pointer to it. 380 381This routine is necessary because each back end has private 382information surrounding the @code{asymbol}. Building your own 383@code{asymbol} and pointing to it will not create the private 384information, and will cause problems later on. 385@example 386#define bfd_make_empty_symbol(abfd) \ 387 BFD_SEND (abfd, _bfd_make_empty_symbol, (abfd)) 388 389@end example 390 391@findex _bfd_generic_make_empty_symbol 392@subsubsection @code{_bfd_generic_make_empty_symbol} 393@strong{Synopsis} 394@example 395asymbol *_bfd_generic_make_empty_symbol (bfd *); 396@end example 397@strong{Description}@* 398Create a new @code{asymbol} structure for the BFD @var{abfd} 399and return a pointer to it. Used by core file routines, 400binary back-end and anywhere else where no private info 401is needed. 402 403@findex bfd_make_debug_symbol 404@subsubsection @code{bfd_make_debug_symbol} 405@strong{Description}@* 406Create a new @code{asymbol} structure for the BFD @var{abfd}, 407to be used as a debugging symbol. Further details of its use have 408yet to be worked out. 409@example 410#define bfd_make_debug_symbol(abfd,ptr,size) \ 411 BFD_SEND (abfd, _bfd_make_debug_symbol, (abfd, ptr, size)) 412 413@end example 414 415@findex bfd_decode_symclass 416@subsubsection @code{bfd_decode_symclass} 417@strong{Description}@* 418Return a character corresponding to the symbol 419class of @var{symbol}, or '?' for an unknown class. 420 421@strong{Synopsis} 422@example 423int bfd_decode_symclass (asymbol *symbol); 424@end example 425@findex bfd_is_undefined_symclass 426@subsubsection @code{bfd_is_undefined_symclass} 427@strong{Description}@* 428Returns non-zero if the class symbol returned by 429bfd_decode_symclass represents an undefined symbol. 430Returns zero otherwise. 431 432@strong{Synopsis} 433@example 434bfd_boolean bfd_is_undefined_symclass (int symclass); 435@end example 436@findex bfd_symbol_info 437@subsubsection @code{bfd_symbol_info} 438@strong{Description}@* 439Fill in the basic info about symbol that nm needs. 440Additional info may be added by the back-ends after 441calling this function. 442 443@strong{Synopsis} 444@example 445void bfd_symbol_info (asymbol *symbol, symbol_info *ret); 446@end example 447@findex bfd_copy_private_symbol_data 448@subsubsection @code{bfd_copy_private_symbol_data} 449@strong{Synopsis} 450@example 451bfd_boolean bfd_copy_private_symbol_data 452 (bfd *ibfd, asymbol *isym, bfd *obfd, asymbol *osym); 453@end example 454@strong{Description}@* 455Copy private symbol information from @var{isym} in the BFD 456@var{ibfd} to the symbol @var{osym} in the BFD @var{obfd}. 457Return @code{TRUE} on success, @code{FALSE} on error. Possible error 458returns are: 459 460@itemize @bullet 461 462@item 463@code{bfd_error_no_memory} - 464Not enough memory exists to create private data for @var{osec}. 465@end itemize 466@example 467#define bfd_copy_private_symbol_data(ibfd, isymbol, obfd, osymbol) \ 468 BFD_SEND (obfd, _bfd_copy_private_symbol_data, \ 469 (ibfd, isymbol, obfd, osymbol)) 470 471@end example 472 473